STIMULI AND THEIR ACTIONS 353 



at the same time very important vital conditions; and these 

 stimuli, which are necessary to the continued maintenance of life, 

 without which the nutrition, the metabolism, of the organs in 

 question cannot continue undisturbed, are termed trophic stimuli. 



Trophic stimuli do not stand in contrast with other stimuli ; 

 the term " trophic " simply signifies a special peculiarity of the 

 action, and very different stimuli can have a trophic effect. As 

 regards trophic stimuli that in the animal organism are transmitted 

 through the nerves to the tissues, it has been believed that 

 special trophic nerve-fibres and nerve-centres must be assumed in 

 addition to the fibres and centres of known function ; such nerve- 

 fibres are asserted to have nothing whatever to do with the 

 peculiar function of the tissue supplied by them, but merely 

 regulate its nutrition and metabolism. This idea of so-called 

 trophic nerves has produced in physiology and medicine much 

 mischief and confusion, and recently has misled many men 

 of science into the most fantastic ideas and supposed discoveries. 

 But for every critical investigator, who is wont to associate a 

 definite idea with the conceptions with which he deals, the con- 

 fused idea of trophic nerves is simply a piece of the old mysticism 

 of the vitalists. It is seen that the assumption of special trophic 

 nerves and peculiar trophic stimuli, existing in addition to other 

 stimuli, is not needed in order to explain the phenomena, but that 

 the nerves that influence the characteristic function of every 

 tissue regulate thereby the metabolism of the cells in question ; 

 in other words, every nerve serves as a trophic nerve for the tissue 

 that it supplies, since the impulse which it conveys represents a 

 vital condition for the tissue. 



B. THE IRRITABILITY OF LIVING SUBSTANCE 



1. The Conception of Irritability and the Nature of Reactions 



Every process of stimulation requires two factors : a stimulus, 

 and a body that is irritable. If the two factors come into 

 correlation there results a phenomenon of stimulation, a reaction. 

 We have considered stimuli ; we will now consider irritability. 



A definition of irritability (excitability) that shall have general 

 application, must be formulated somewhat as follows : The irrita- 

 bility of living substance is its capacity of reacting to changes in its 

 environment by changes in the equilibrium of its matter and its 

 energy. All other factors that might be included in the definition 

 would be applicable to special cases only. Yet, frequently, the 

 general conception, without being exactly defined, has more or 

 less unconsciously been made to include special factors. For 

 example, as regards the quantitative relations of the stimulus and 

 the reaction, that case has been regarded as the type in which an 



A A 



